THIS IS A SAMPLE OF THE KIND OF ARTICLES INCLUDED IN MY “THOMPSON TECHNICAL” BOOKS
THOMPSON TECHNICAL
By D.W. Richardson
MODELS INTRODUCTION
(copyright 1995)
There are two basic types of Thompson Submachine Guns. They are the 1921/1928/1928A1 models and the M1/M1A1 models.
The first Thompson model put into volume production was the 1921. This was the “gangster” gun. They were all made by Colt and were finished with a rust blue. With minor modifications it became the 1928 and then the 1928A1. These are essentially all the same gun and all parts interchange between all 1921 through 1928A1 models. By changing minor parts one model becomes another. The recognizable features include a receiver which is rectangular in cross sectional shape with slots to accept a drum magazine and a top mounted bolt handle. The receiver also incorporates provisions for a bolt delay mechanism to retard the movement of the bolt immediately after firing. The trigger housing is fitted with a slot to accept the drum magazine and rails to slide on a detachable buttstock. These guns are usually fitted with a Lyman adjustable rear sight, finned barrel, and often a Cutts compensator. They may also be fitted with either a vertical or horizontal front grip. The vertical front grip was standard on the 1921 and optional on the 1928. The horizontal front grip was optional on the 1928 and standard on the 1928A1. The 1921-1928A1 guns all saw action during WWII although the standard U.S. Army configuration was the 1928A1. Thompsons made during WWII were manufactured by the Auto-Ordnance Corp. of Bridgeport Connecticut and the Savage Arms Co., although all WWII guns were marked “Auto-Ordnance Corp.”. All WWII manufactured Thompsons were finished in black oxide.
Early in WWII, the Thompson gun was redesigned to create the Model M1 and, subsequently, the Model M1A1. The distinctive features of these are the side mounted bolt handle, fixed buttstock, and a “hat” shaped receiver cross sectional shape with no provisions for a drum. These models deleted the bolt delay mechanism to create a purely slam-fire operation. The rear sight is a non-adjustable sheet metal stamping. Compensators and finned barrels were not fitted. The major difference between the M1 and M1A1 is the bolt which is interchangeable. The M1 bolt incorporates a firing pin mechanism similar to the 1921-1928A1 guns whereas the M1A1 bolt has a protrusion machined onto the bolt face to act as a firing pin. The M1A1 was the last model Thompson gun to be produced. All production of Thompson guns was terminated in 1944. Shortly thereafter the company went out of the gun business.
Many similarities exist among all models of Thompson guns and most parts will interchange. All original production Thompson Submachine Guns are chambered for the .45 Automatic Colt Pistol (ACP) cartridge. All models of trigger housings will fit on all models of receivers. However, the rear end shape of the 1921-1928A1 differs from the M1/M1A1, so there is a mismatch when trigger housings are interchanged. All types of barrels, grip mounts, and front grips fit all Thompson receivers. However, the muzzle end of the barrel is a smooth taper when configured for a ring type sight and threaded when a compensator is fitted. The taper can be threaded for a compensator but a ring type sight can not be fitted to a threaded barrel.
It is not practical to modify an M1/M1A1 to accept a detachable buttstock or drum magazine. The trigger housing components are virtually the same for all models except for the rear grip and magazine catch which are different for the M1/M1A1. The M1/M1A1 magazine catch does not have a provision for drum magazines, however, most M1/M1A1 guns are fitted with 1928A1 style magazine catches. None of the receiver bolt cavity components can be interchanged between 1921-1928A1 and M1/M1A1 guns except for the extractor; and hammer pin, firing pin, and firing pin spring (M1 only). The recoil spring is the same for 1928/1928A1 and M1/M1A1 guns. The 1921 recoil spring is unique. All rear sights interchange but the Lyman adjustable rear sight will overhang the M1/M1A1 receiver top edge radii. Buttstock components are not interchangeable with the exception of the large buttplate screw, sling swivel and screws, and buttstock mounting bolts. Ejectors are not interchangeable between 1921-1928A1 and M1/M1A1 guns.
MODELS M1 & M1A1 THOMPSON SUBMACHINE GUNS